Cysteine (—S—S—) bridges are common structural motifs in naturally occurring cyclic peptides. In some cases, these disulfide bridges act as reactive functional groups. In many other cases however, the cysteine bridge serves a skeletal, structural role, maintaining secondary and tertiary structure. Disulfide bonds in peptides and other compounds are highly reactive under broad-ranging conditions, and therefore useful peptides containing disulfide bonds that have a structural role are at risk of denaturation, resulting in loss of properties.